In ball joints equipped in various apparatuses such as vehicles, a dust cover is used for preventing entrance of water, dust or the like into the joint part and for preventing escape of grease from the joint part. A dust cover used in a ball joint according to a prior art example will be described with reference to FIG. 6. FIG. 6 is a schematic cross-sectional view of the ball joint and the dust cover mounted in the ball joint according to the prior art example.
The ball joint includes a ball stud 300 that has a spherical part at one end of a shaft part thereof, a socket 200 that supports the ball stud 300 rotatably and swingably, and a knuckle 400 provided on the opposite side of the shaft part from the spherical part. The dust cover 700 includes a sealing body made of an elastic material and integrally having a film-like deformable body part 710, a fixed part 730 provided at one end of the body part 710 to be fixed to the socket 200, and a sealed part 720 provided at the other end of the body part 710. The sealed part 720 includes an inner circumferential sealed portion 721 that is slidable against the shaft part of the ball stud 300, and a dust lip 722 that is slidable against the knuckle 400. A reinforcing ring 750 made of metal or resin is embedded in the sealed part 720.
The reinforcing ring 750 includes a tubular part that is concentric to the shaft part of the ball stud 300. The inner circumferential sealed portion 721 is pressed radially inward by this tubular part. This way, the sealing function provided by the inner circumferential sealed portion 721 is consistently exhibited. However, with the dust cover 700 according to the prior art example, the sealing function of the dust lip 722 is dependent on the deformed state of the body part 710, because of which the sealing performance is not consistently achieved. Namely, when the ball stud 300 is inclined to the left in the drawing as shown in FIG. 6, for example, the portion of the body part 710 on the left side of the drawing is compressed, while the portion on the right side of the drawing is stretched. Therefore, while the portion of the dust lip 722 on the left side of the drawing is pressed against the knuckle 400 and can therefore fully provide the sealing function, the portion on the right side of the drawing is subjected to a force that causes the lip to move away from the knuckle 400, because of which the sealing function is compromised. The sealing function provided by the dust lip 722 is not necessarily consistent in this way.